The present invention generally relates to medical methods and systems, particularly for the detection of unstable oxygen saturation of a patient.
Obstructive sleep apnea is recognized as one of the most common disorders in the United States. The lower oxygen levels associated with obstructive sleep apnea play a major role in cardiovascular morbidity including cardiac arrest and stroke. Obstructive sleep apnea causes characteristic patterns of gradual oxygen de-saturation, followed by rapid re-saturation when a sleeping patient's body manages to briefly increase muscle tone in the upper airway to sufficiently resume respiration. This pattern tends to repeat cyclically at roughly similar periods and saturation amplitudes. Millions of patients experience such characteristics of sleep apnea for years at a time without being diagnosed or treated as acute health consequences do not readily arise. Despite the fact that obstructive sleep apnea is easily treated, both the patient and the family are often completely unaware of the presence of this dangerous condition as anything more than heavy snoring.
Obstructive sleep apnea often develops as patients enter middle age and begin to snore. The major cause is an increase in fat deposition in the neck which results in narrowing of the airway. When the muscle tone of the upper airway diminishes during sleep, negative pressure associated with inspiration through this narrow airway results in collapse of the upper airway which effectively chokes off all air movement and a fall in oxygen. The fall in oxygen produces central nervous system stimulation contributing to hypertension, potential heart and blood vessel injury, and finally results in arousal of the patient. Upon arousal, increase in airway muscle tone opens the upper airway and the patient rapidly inhales and ventilates quickly to correct the low oxygen levels. Generally, the arousal is brief and the patient is not aware of the arousal.
Undiagnosed obstructive sleep apnea can lead to serious consequences, including progressive decline in heart muscle function, blood vessel damage, and even death by stroke or cardiac arrest.
Detection of such profound physiologic instability may be accomplished by conventional polysomnography. However, this approach is expensive and difficult to implement on a sufficient scale. Further diagnosis approaches for detecting obstructive sleep apnea with the use of oximetry systems are described by the following U.S. Pat. Nos. 5,398,682; 5,605,151; 5,891,023; 6,223,064; 6,342,039; 6,609,016; 6,748,252; 6,760,608 and U.S. Patent Publication Nos. 2002/0190863; 2003/0000522; 2003/0158466, the full disclosures of which are incorporated herein by reference. A number of these references are directed to analyzing waveshapes of various signals.
Alternative diagnostic methods and systems for identifying sleep apnea that do not require evaluation of a patient by polysomnography or the analysis of a succession of specific events could be advantageous.